Newly established assay method for 25-hydroxyvitamin D3 24-hydroxylase revealed much lower Km for 25-hydroxyvitamin D3 than for 1alpha,25-dihydroxyvitamin D3.

An accurate assay method of 25-hydroxyvitamin D3 24-hydroxylase (24-hydroxylase) was established. Kidney mitochondria prepared from vitamin D-replete rats were treated with polyoxyethylenesorbitan monolaurate. The solubilized suspension was ultracentrifuged at 100,000g for 60 minutes and an aliquot of the supernatant was incubated under the saturating concentrations of substrate NADPH and the mitochondrial-type electron transferring proteins, adrenodoxin and NADPH-adrenodoxin reductase. Products were analyzed by high-performance liquid chromatography (HPLC) monitoring effluents at a wavelength of 265 nm. The maximal velocity of the enzyme in vitamin D-replete rats was 400 pmol/minute per mg of protein, which was considerably higher than those reported by previous authors who used intact kidney mitochondria as the enzyme source. In applying the new assay method, an interesting property was found; Michaelis constant of 24-hydroxylase for 25-hydroxyvitamin D3 [25(OH)D3] was 0.6 microM, which was 35-fold lower than that for 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] which was 20.9 microM. This fact indicates that affinity of the enzyme to 25(OH)D3 is 35-fold higher than that to 1alpha,25(OH)2D3. These data suggest that 25(OH)D3 is the preferred substrate to 1alpha,25(OH)2D3.     

Serum concentrations of 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D in patients with various types of renal disease

Serum concentrations of 25-hydroxyvitamin D (25OHD) and 24,25-dihydroxyvitamin D (24,25(OH)2D) in patients with various types of renal disease were measured by a competitive protein binding assay. There was a significant (P less than 0.001) inverse correlation between serum levels of either 25OHD or 24,25(OH)2D and the degree of proteinuria in patients with chronic glomerulonephritis or idiopathic nephrotic syndrome. The ratio of 24,25(OH)2D to 25OHD was relatively low in patients with creatinine clearances (CCr) less than 30 ml/min/1.48 m2, while the ratio was higher in those with clearances greater than 85 ml/min/1.48 m2. There was a linear correlation (r = 0.783, P less than 0.001) between the ratio and the CCr in patients whose CCR ranged from 30 to 85 ml/min/1.48 m2. The 24,25(OH)2D/25OHD ratio also appeared to be correlated significantly (P less than 0.001) with the PSP-test. The serum levels of 25OHD and 24,25(OH)2D were lowered in nephrotic patients during treatment with prednisolone. The serum levels of 24,25(OH)2D were increased by 1 alpha-hydroxyvitamin D3 treatment in patients with chronic renal failure.     

Renal function and 25-hydroxyvitamin D concentrations predict parathyroid hormone levels in renal transplant patients.

BACKGROUND: Recent guidelines suggest supplementation with ergocalciferol (vitamin D(2)) in chronic kidney disease stages 3 and 4 patients with elevated parathyroid hormone (PTH) levels and 25-hydroxyvitamin D (25OHD) levels <75 nmol/l. These guidelines are also applied to renal transplant patients. However, the prevalence rates of 25OHD deficiency and its association with PTH levels in renal transplant populations have not been extensively examined. We aimed to document the prevalence rates of 25OHD deficiency [defined by serum levels <40 nmol/l (<16 ng/ml)] and insufficiency [<75 nmol/l (<30 ng/ml)] in a single renal transplant centre, and examine its relationship with PTH levels. METHODS: Serum 25OHD and PTH concentrations were measured in 419 transplant patients attending a single renal transplant clinic over a 4-month period. Demographic and biochemical data were also collected, including serum creatinine, calcium, phosphate and albumin. Simple and multiple linear regression analysis were performed. RESULTS: In 27.3% of the patients, 25OHD deficiency was present, and 75.5% had insufficiency. On univariate analysis, 25OHD, serum albumin and estimated glomerular filtration rate (eGFR) were significantly associated with PTH levels (P < 0.0001, P = 0.004 and P < 0.0001, respectively). Multiple linear regression demonstrated that only 25OHD, eGFR and serum phosphate were significantly predictive of PTH levels (R(2) = 0.19, P < 0.0001). In this model, a 75 nmol/l increase in 25OHD will only result in a maximal reduction in PTH of 2.0 pmol/l. CONCLUSIONS: We conclude that 25OHD deficiency and insufficiency are common in renal transplant patients and may exacerbate secondary hyperparathyroidism. However, 25OHD, eGFR and phosphate only account for 19% of the variability in PTH levels. In addition, even a large increase in serum 25OHD levels is likely to result in only a small reduction in PTH. Therefore, alternative approaches to managing hyperparathyroidism in renal transplant recipients rather than supplementation with ergocalciferol are warranted.     

Expression of 25-hydroxyvitamin D3-1alpha-hydroxylase in the human kidney

The secosteroid hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) plays a vital role in calcium metabolism, tissue differentiation, and normal bone growth. Biosynthesis of 1,25(OH)2D3 is catalyzed by the mitochondrial cytochrome P450 enzyme 25-hydroxyvitamin D3 1alpha-hydroxylase (1alpha-hydroxylase). Although activity of this enzyme has been described in several tissues, the kidneys are recognized to be the principal site of 1,25(OH)2D3 production. To date, enzyme activity studies using vitamin D-deficient animals have suggested that 1alpha-hydroxylase is expressed exclusively in proximal convoluted tubules. With the recent cloning of 1alpha-hydroxylase, specific cRNA probes and in-house polyclonal antiserum have been used to determine the distribution of 1alpha-hydroxylase along the human nephron. Immunohistochemistry and in situ hybridization studies indicated strong expression of 1alpha-hydroxylase protein and mRNA in the distal convoluted tubule, the cortical and medullary part of the collecting ducts, and the papillary epithelia. Lower expression was observed along the thick ascending limb of the loop of Henle and Bowman's capsule. Weaker and more variable expression of 1alpha-hydroxylase protein and mRNA was seen in proximal convoluted tubules, and no expression was observed in glomeruli or vascular structures. These data show for the first time the distribution of alpha1-hydroxylase expression in normal human kidney. In contrast to earlier enzyme activity studies conducted in vitamin D-deficient animals, our data indicate that the distal nephron is the predominant site of 1alpha-hydroxylase expression under conditions of vitamin D sufficiency.     

25-hydroxyvitamin D in patients with essential hypertension

Plasma 25-OH-D levels of normotensive and hypertensive subjects living in a highly industrial and in a nonindustrial area, were estimated. Significantly higher values were found in normotensive subjects than in hypertensive patients. Severity of arterial hypertension was not related to plasma 25-OH-D. Both groups of subjects (normotensive and hypertensive) living in the industrial area with high air pollution had lower 25-OH-D concentrations than those living in the nonindustrial region. Data presented in this paper suggest that antihypertensive treatment by pharmacologic agents exerts a depressive effect on plasma 25-OH-D level.     

Expression of 25-hydroxyvitamin D3-1alpha-hydroxylase along the nephron: new insights into renal vitamin D metabolism

Renal synthesis of the active form of vitamin D, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], is a pivotal step in calcium and phosphate homeostasis. Production of 1,25(OH)2D3 is catalyzed by the mitchondrial cytochrome P450, 25-hydroxyvitamin D3-1alpha-hydroxylase (1alpha-HYD). As a consequence of the tight regulation of vitamin D metabolism during normal physiology, studies of the expression and regulation of 1alpha-HYD have proved remarkably difficult. However, the recent cloning of the gene for 1alpha-HYD has enabled a more comprehensive analysis of the tissue distribution of 1alpha-HYD, as well as the mechanisms involved in controlling 1,25(OH)2D3 production. In particular, an understanding of site-specific expression and regulation of 1alpha-HYD along the nephron might help to elucidate a more versatile role for 1,25(OH)2D3 in renal physiology.     

Relationship between serum 25-hydroxyvitamin D3 and heart valvular calcification in chronic kidney disease patients stage 3-5

Objective To detect the prevalence of heart valvular calcification (VC) and its related risk factors, and to investigate correlation between serum 25-hydroxyvitamin D3[25(OH)D3] and VC in chronic kidney disease (CKD) stage 3-5 patients. Methods A total of 294 CKD patients stage 3-5 were admitted in The Second Affiliated Hospital of Anhui Medical University. Their clinical and laboratory data were collected, patients were classified into two groups according echocardiography: patients with VC were defined as VC group while others were defined as non-VC group. The differences of 25(OH)D3 level and other data in two group were assessed, and related risk factors of VC were analyzed. Results Among 294 CKD patients, 82 were with VC (27.9%) while 212 were without VC (72.1%); serum 25(OH)D3 level was significantly higher in VC group than in non-VC group [(11.9±9.3) μg/L vs (9.6±7.2) μg/L, P＜0.05]. Age, cystatin C, hypersensitive C-reactive protein, pulmonary artery pressure, proportion of secondary hyperparathyroidism, incidence of abdominal aortic calcification and taking active vitamin D proportion were higher in VC group than in non-VC group (P＜0.05). Two classification logistic regression analyses showed that advanced age, high intact parathyroid hormone (iPTH) and 25(OH)D3, pulmonary arterial hypertension were risk factors for VC in CKD stage 3-5 patients. Conclusions The prevalence of VC is high in CKD stage 3-5 patients. Advanced age, bone metabolic disorder and pulmonary arterial hypertension are associated with VC.     

慢性肾脏病5期患者25-羟维生素D3不足与缺乏

目的 评估慢性肾脏病(CKD)5期患者25-羟维生素D3[25 (OH) D3]不足与缺乏的患病率及其影响因素.方法 对本院96例CKD 5期患者的病史、实验室检查结果等进行回顾性分析.纳入研究的变量包括:血25 (OH) D3检测值,血白细胞(WBC)、血红蛋白(Hb)、血清尿素氮(BUN)、血清肌酐(Scr)、二氧化碳结合力(CO2CP)、血清白蛋白(Alb)、碱性磷酸酶(ALP)、总胆固醇(TCH)、高密度脂蛋白(HDL-C)、低密度脂蛋白(LDL-C)、甘油三脂(TG)、钙、磷、全段甲状旁腺激素(iPTH)等.分析25 (OH) D3水平与各项观察指标间的关系.结果 96例CKD 5期患者的25 (OH) D3平均水平为33.25(24.85～ 44.30) nmol/L,显著低于正常值(P＜0.01);非透析患者、维持性血液透析(以下简称血透)患者、维持性腹膜透析(以下简称腹透)患者25-羟维生素D3水平分别为32.70(25.30～43.70) nmol/L、37.00(29.20～ 48.65)nmoL/L和27.05(19.20 ～ 35.37) nmol/L.CKD5期患者的25 (OH) D3不足患病率为32.29％;在非透析、血透、腹透患者中分别为27.91％、45.45％和20％;CKD5期患者25 (OH) D3缺乏患病率为64.58％,在非透析、血透、腹透患者中分别为67.44％、51.52％和80％;25 (OH) D3缺乏及不足患病率为96.88％,非透析、血透、腹透患者中分别为95.35％、96.97％和100％,各患病率三组间差异无统计学意义.单因素相关分析结果显示,25 (OH) D3水平与Hb(r=0.222)、Alb(r=0.398)相关(P＜0.05).多元线性回归分析结果显示,Alb水平与25 (OH) D3水平呈正相关.结论 CKD5患者的维生素D缺乏和不足患病率高,普遍存在.Alb是CKD5期患者维生素D水平不足或缺乏的独立影响因素.     

Effects of vitamin D compounds on renal and intestinal Ca2+ transport proteins in 25-hydroxyvitamin D3-1alpha-hydroxylase knockout mice

BACKGROUND: Vitamin D compounds are used clinically to control secondary hyperparathyroidism (SHPT) due to renal failure. Newer vitamin D compounds retain the suppressive action of 1,25(OH)(2)D(3) on the parathyroid glands and may have less Ca(2+)-mobilizing activity, offering potentially safer therapies. METHODS: This study investigated the effect of a single dose of compound (1,25(OH)(2)D(3), 1,24(OH)(2)D(2), or 1alpha(OH)D(2)) on renal and intestinal Ca(2+) transport proteins, including TRPV5 and TRPV6, and serum Ca(2+), in a novel SHPT model, the 25-OH-D(3)-1alpha-hydroxylase knockout mouse, which lacks endogenous 1,25(OH)(2)D(3) and is severely hypocalcemic. Animals were injected intraperitoneally with compound (100 ng/mouse). RESULTS: Serum levels of 1,25(OH)(2)D(3) and 1,24(OH)(2)D(2) peaked at four hours post-injection (pi), then declined rapidly. 1,25(OH)(2)D(2) generated from 1alpha(OH)D(2) peaked at 12 hours pi and then remained stable. Serum Ca(2+) was increased to near-normal within four hours by 1,25(OH)(2)D(3) and 1,24(OH)(2)D(2), and within 12 hours by 1alpha(OH)D(2). 1,25(OH)(2)D(3) and 1,24(OH)(2)D(2) up-regulated duodenal TRPV5 and TRPV6 mRNA to a similar degree within four hours; mRNA levels decreased by 12 hours after 1,24(OH)(2)D(2) treatment, and by 24 hours after 1,25(OH)(2)D(3) treatment. 1,25(OH)(2)D(3) increased kidney levels of TRPV5, calbindin-D(28K), and calbindin-D(9K) mRNA within four hours; 1,24(OH)(2)D(2) did not change kidney TRPV5 levels and modestly increased calbindin D(9K) by 48 hours. 1alpha(OH)D(2) produced later-onset effects, increasing duodenal TRPV6 and calbindin-D(9K) mRNA levels by 12 hours and TRPV5 by 48 hours. CONCLUSION: In kidney, 1alpha(OH)D(2) increased TRPV5, calbindin-D(28K), and calbindin-D(9K) mRNA levels by 12 hours. This study indicates that Ca(2+) transport proteins, including TRPV5 and TRPV6, are differentially up-regulated by vitamin D compounds.     

Seasonal variation of serum 25-hydroxyvitamin D levels in adult patients with inflammatory bowel disease

Summary  

Patients with inflammatory bowel disease (IBD) are at risk of osteoporosis. Vitamin D (vitD) deficiency is known as a risk factor of osteoporosis. We observed low vitD blood levels in adult IBD patients both at the end of summer and winter. Furthermore, effects of oral vitD supplementation in (generally low) daily dosages were poor.     

Identification of 25-hydroxyvitamin D3 1alpha-hydroxylase gene expression in macrophages

BACKGROUND: The 25-hydroxyvitamin D3 1alpha-hydroxylase (1alpha-hydroxylase) is almost exclusively expressed in the kidney. However, 1alpha-hydroxylase activities have been observed in some extrarenal tissues, including inflammatory cells of the monocyte/macrophage lineage. In sarcoidosis, macrophage 1alpha-hydroxylase causes overproduction of 1,25-(OH)2D3, resulting in hypercalcemia. In this study, we investigated the regulation of macrophage 1alpha-hydroxylase at a molecular level. METHODS: We used the human monocytic cell line THP-1, which can be differentiated into macrophage-like cells by treatment with phorbol ester. The expression of 1alpha-hydroxylase in THP-1 cells was examined by Northern blotting and immunoblotting using an antibody raised against a synthetic peptide corresponding to the 14 C-terminal amino acids of 1alpha-hydroxylase. We investigated the regulation of 1alpha-hydroxylase mRNA expression by RNase protection assay. RESULTS: Northern blot and immunoblot analyses confirmed the expression of 1alpha-hydroxylase in THP-1 cells at the mRNA and protein levels. Although parathyroid hormone and calcitonin, known stimulators of renal 1alpha-hydroxylase, did not affect the expression of 1alpha-hydroxylase mRNA, 8-Br-cAMP (5 x 10-4 mol/L) increased the expression of 1alpha-hydroxylase mRNA in THP-1 cells (198 +/- 9%). 1,25-(OH)2D3, known as a suppressor of renal 1alpha-hydroxylase, did not affect the expression of 1alpha-hydroxylase mRNA. By contrast, 1,25-(OH)2D3 markedly increased the expression of 25-hydroxyvitamin D3 24-hydroxylase mRNA. Interferon-gamma (2000 IU/mL) increased the expression of 1alpha-hydroxylase mRNA in differentiated THP-1 cells (922 +/- 25%). CONCLUSIONS: The present results suggest that 1alpha-hydroxylase activity in macrophages is mediated by the same enzyme as in kidney. Interferon-gamma treatment increases macrophage 1alpha-hydroxylase levels via directly increasing gene expression of this enzyme.     

The renal metabolism of 25-hydroxyvitamin D3 in the rat: Regulation by 1,25-dihydroxyvitamin D3

Summary To determine the effects of 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] on the renal metabolism of 25-hydroxyvitamin D₃ [25(OH)D₃], the influence of 1,25(OH)₂D₃ and 24,25-dihydroxyvitamin D₃ [24,25(OH)₂D₃] was compared in vitamin D-deficient rats. Serum calcium (Ca), serum immunoreactive parathyroid hormone (iPTH) and the specific activities (SA) of renal 25(OH)D₃: 24-hydroxylase (24-hydroxylase) and 25(OH)D₃: 1α-hydroxylase (1-hydroxylase) were measured. In vitamin D-deficient rats, mean serum Ca was low, serum iPTH was increased, renal 1-hydroxylase was increased, and renal 24-hydroxylase was below the limit of detection. Treatment with either 1,25(OH)₂D₃ or 24,25(OH)₂D₃, 50 ng/d for 2 days, significantly increased mean serum Ca but did not change serum iPTH, renal 1-hydroxylase SA, or renal 24-hydroxylase SA. 1,25(OH)₂D₃, 50 ng/d for 7 days, returned serum Ca and iPTH to normal, lowered renal 1-hydroxylase SA, and induced renal 24-hydroxylase activity. In contrast, 24,25(OH)₂D₃, 50 ng/d for 7 days, similarly lowered renal 1-hydroxylase SA but did not induce renal 24-hydroxylase activity. Thyroparathyroidectomy of vitamin D-deficient rats resulted in a rapid decline in 1-hydroxylase SA. The results indicate that in vitamin D-deficient rats a) 1,25(OH)₂D₃ reduces renal 1-hydroxylase SA and increases renal 24-hydroxylase SA and b) 24,25(OH)₂D₃ reduces renal 1-hydroxylase SA and does not alter renal 24-hydroxylase SA. Inhibition of renal 1-hydroxylase by the two metabolites is apparently mediated through changes in serum Ca and circulating iPTH, whereas stimulation by 1,25(OH)₂D₃ of renal 24-hydroxylase activity is direct.     

Serum 25-hydroxyvitamin D in patients with chronic renal failure on long-term treatment with high doses of vitamin D2

The present study was aimed at answering the following two questions: (1) What is the effect of high dose vitamin D treatment on the serum level of 25-hydroxyvitamin D (25-OH-D) in patients with chronic renal failure (CRF)? (2) Is there any effect of urinary protein loss on the serum 25-OH-D levels during treatment with pharmacological doses of vitamin D? 42 patients with CRF were studied. They were treated conservatively by a low protein diet and received 15 mg of vitamin D2 once a week. Long-term administration of vitamin D caused a significant (5- to 7-fold) increase of plasma 25-OH-D level irrespective of the degree of proteinuria. This increase was noted only during the first 5 months of vitamin D2 treatment. Surprisingly only in some patients moderate hypercalcemia (> 2.75 mmol/l) was found. From the results obtained it is concluded that (1) patients with CRF differ from normal subjects in handling of high doses of vitamin D and (2) high dosage treatment with vitamin D may prevent hypocalcemia in patients with CRF in spite of high proteinuria.     

Effects of prostaglandin E2 and indomethacin on 25-hydroxyvitamin D3-1α-hydroxylase activity in isolated kidney cells of normal and streptozotocin-induced diabetic rats

Summary The effects of prostaglandin E₂ (PGE₂) (1 μM) and indomethacin (IN) (20 μM) on 1,25 dihydroxyvitamin D₃ production were studied in renal cell suspensions isolated from control, stroptozotocin-induced diabetic, and insulin-treated diabetic rats. Renal cortex cells were isolated by enzymatic digestion, and preincubated for 30 min at 37°C with the appropriate additive(s) followed by a 1 h incubation with 8 nM 25-hydroxyvitamin D₃ in serumfree medium. Radioactivity incorporated into that fraction of the cell suspension extract co-eluting with synthetic 1,25-dihydroxyvitamin D₃ on high pressure liquid chromatography was determined All animals were raised for 5 weeks on a vitamin D-deficient diet. Isolated kidney cells from vitamin D-deficient rats showed dose-dependent response of 1,25 dihydroxyvitamin D₃ production to PGE₂. Cells from control animals demonstrated a stimulatory effect of PGE₂ (P<0.05) and a suppressive effect of IN (P<0.01) on 1,24 dihydroxyvitamin D₃ production. In contrast, cells from diabetic rat kidneys failed to respond to these agents, alterations which were reversed by insulin treatment. The accumulated data suggest that depressed synthesis of 1,24 dihydroxyvitamin D₃ previously observed in the experimental diabetic rat is due, at least in part, to an impaired production and response to PGE₂-like prostaglandins.     

25-hydroxyvitamin D levels and bone histomorphometry in hemodialysis renal osteodystrophy

BACKGROUND: The importance of 25-hydroxyvitamin D (25-OHD) serum levels in hemodialysis chronic renal failure has not been so far histologically evaluated. Information still lacking relate to the effect of 25-OHD deficiency on serum parathyroid hormone (PTH) levels and on bone and its relationship with calcitriol levels. METHODS: This retrospective study has been performed on a cohort of 104 patients on hemodialysis from more than 12 months, subjected to transiliac bone biopsy for histologic, histomorphometric, and histodynamic evaluation. The patients, 61 males and 43 females, mean age 52.9 +/- 11.7 years, hemodialysis length 97.4 +/- 61.4 months, were treated with standard hemodialysis and did not receive any vitamin D supplementation. Treatment with calcitriol was not underway at the time of the biopsy. Transiliac bone biopsies were performed after double tetracycline labels. In addition, serum intact PTH (iPTH), alkaline phosphatase, and 25-OHD were measured. Calcitriol serum levels was also measured in a subset of patients (N= 53). The patients were divided according to serum 25-OHD levels in three groups: (1) 0 to 15 (15 patients), (2) 15 to 30 (38 patients), and (3) >30 ng/mL (51 patients). RESULTS: There was no significant difference in average age, hemodialysis age, serum PTH [490 +/- 494, 670 +/- 627, and 489 +/- 436 pg/mL, respectively (mean +/- SD)], alkaline phosphatase, and calcitriol between the three groups. The parameters double-labeled surface, trabecular mineralizing surface, and bone formation rate were significantly lower in group 1 than in the other groups (P < 0.03, < 0.03, and < 0.02, respectively). Osteoblast surface and adjusted apposition rate were borderline significantly lower in group 1 (P < 0.06 and < 0.10). There was no statistical difference in the biochemical and bone parameters between groups 2 and 3. A positive significant correlation was found between several bone static and dynamic parameters and 25-OHD levels in the range 0 to 30 ng/mL, showing a vitamin D dependence of bone turnover at these serum levels. However, actual evidence of an effect on bone of 25-OHD deficiency was found at serum levels below 20 ng/mL. With increasing 25-OHD levels beyond 40 ng/mL, a downslope of parameters of bone turnover was also observed. CONCLUSION: Since PTH serum levels are equally elevated in low and high 25-OHD patients, while calcitriol levels are constantly low, an effect of 25-OHD deficiency (group 1) on bone, consisting of a mineralization and bone formation defect, can be hypothesized. The effect of vitamin D deficiency or bone turnover is found below 20 ng/mL. The optimal level of 25-OHD appears to be in the order of 20 to 40 ng/mL. Levels of the D metabolite higher than 40 ng/mL are accompanied by a reduction of bone turnover.     

Serum levels of 1,25-dihydroxyvitamin D, 24,25-dihydroxyvitamin D, and 25-hydroxyvitamin D in nondialyzed patients with chronic renal failure

BACKGROUND: In patients with chronic renal failure (CRF), abnormalities in vitamin D metabolism are known to be present, and several factors could contribute to the abnormalities. METHODS: We measured serum levels of three vitamin D metabolites, 1,25(OH)2D, 24, 25(OH)2D and 25(OH)D, and analyzed factors affecting their levels in 76 nondialyzed patients with CRF (serum creatinine> 1.6 and < 9.0 mg/dl), 37 of whom had diabetes mellitus (DM-CRF) and 39 of whom were nondiabetic (nonDM-CRF). RESULTS: Serum levels of 1,25(OH)2D were positively correlated with estimated creatinine clearance (CCr; r = 0.429; P < 0.0001), and levels of 24,25(OH)2D were weakly correlated with CCr (r = 0.252, P < 0.05); no correlation was noted for 25(OH)D. Serum levels of all three vitamin D metabolites were significantly and positively correlated with serum albumin. Although there were no significant differences in age, sex, estimated CCr, calcium and phosphate between DM-CRF and nonDM-CRF, all three vitamin D metabolites were significantly lower in DM-CRF than in nonDM-CRF. To analyze factors influencing vitamin D metabolite levels, we performed multiple regression analyses. Serum 25(OH)D levels were significantly and independently associated with serum albumin, presence of DM and serum phosphate (R2 = 0.599; P < 0.0001). 24,25(OH)2D levels were significantly and strongly associated with 25(OH)D (beta = 0.772; R2 = 0.446; P < 0.0001). Serum 1,25(OH)2D levels were significantly associated only with estimated CCr (R2 = 0. 409; P < 0.0001). CONCLUSIONS: These results suggest that hypoalbuminemia and the presence of DM independently affect serum 25(OH)D levels, probably via diabetic nephropathy and poor nutritional status associated with diabetes, and that 25(OH)D is actively catalyzed to 24,25(OH)2D in CRF, probably largely via extrarenal 24-hydroxylase. Serum levels of 1,25(OH)2D were significantly affected by the degree of renal failure. Thus, this study indicates that patients with CRF, particularly those with DM, should receive supplements containing the active form of vitamin D prior to dialysis.